Ceramics have a polycrystalline microstructure possessing superior mechanical properties and in particular strength, wear resistance and stability in aggressive and high temperature environments. These mechanical properties make ceramics a desired material for the production of many applications, such as armor, lining of wear and friction surfaces, metal cutting tools, and many other industrial, medical and abrasive applications.
A wider use of ceramics, and in particular alumina, is limited by the fact that it is brittle, which is a general problem common to all ceramics. For example, the fracture strength in bending of conventional alumina ranges from 250-400 MPa. Worse, the Weibull modulus is often low, meaning a large variation in the mechanical strength.
In order to increase the fracture strength of ceramics, secondary materials, such as graphite fibers or silicon carbide fibers, are dispersed throughout the ceramic matrix. Nano-composites, where ceramics act as a matrix and the reinforcing phase is made of submicron metal particles, such as Ni, Ti, Cu, Cr, Co, Mo, W, etc., or ceramics particles, such as SiC, ZrO2, B4C, WC etc., are also known in the art.
Several preparation methods, such as simple powder mixing, oxide reduction, and salt infiltration exist to prepare ceramic matrix nanocomposite. Sintering can be done by pressureless sintering, hot pressing, hot isostatic pressing (HIP), spark plasma sintering (SPS) and other methods.
It has been experimentally shown that crack formation and/or propagation in ceramic nanocomposite can be retarded by stress fields which arise from the mismatch in the coefficient of thermal expansion (CTE) between the matrix and the submicron particles. Flaws at grain boundaries or in the ceramic grains, which expand, are a major source of cracks and, thus, of mechanical failure in ceramics. It is known that if the growth and/or motion of flaws at grain boundaries or in the ceramic grains is retarded, the ceramics' strength and wear resistance will increase.
The submicron or nanometer length-scale reinforcing particles are typically randomly located within the ceramic matrix. Some of the particles are occluded within the grains of the ceramic matrix, while other particles may be located along the grain boundaries or at multi-grain junctions. Occluded particles with a coefficient of thermal expansion greater than that of the matrix material may reduce the matrix strength. For example, Ni particles occluded in an alumina matrix grain; Fe particles occluded in an alumina matrix grain; or Cu particles occluded in an alumina matrix grain.